Linear transformation construction method and system based on Feistel

The invention provides a Feistel-based linear transformation construction method and system, which is applied to a diffusion layer of a cryptographic algorithm, and comprises the following steps: step 1, defining a linear transformation operation A with a branch number of 4; step 2, respectively inputting two inputs X0 and X1 based on the Feistel cipher structure into linear transformation operation A output to obtain A (X0) and A (X1), and carrying out XOR operation on the two outputs to obtain an intermediate variable T; 3, performing cyclic shift on the intermediate variable T, and performing exclusive-or operation on the intermediate variable T and A (X1) to obtain an output Y1 corresponding to the input X1; and performing cyclic shift on the output Y1, and performing exclusive-or operation on the output Y1 and the intermediate variable T to obtain output Y0 corresponding to the input X0, thereby completing linear transformation. According to the method, the optimal branch number in the known public resul